In order to execute chainHead_unstable_call on a block provided by the chainHead_unstable_follow subscription, the user should provide the runtimeUpdates flag set true.
If the runtimeUpgrades flag is set, then the substrate will provide additional details to help users identify if a runtime update happened.

Users that are not interested in runtime upgrades could perform chainHead_unstable_call directly. Upon a breaking change, the user is responsible to fetch and retry with appropriate parameters.

Considering that updates are not that frequent, what would be the impact of allowing users to call into runtime via chainHead_unstable_call regardless of whether the runtimeUpdates is set or not?

I'd expect this to have some implications on the light client side. Would it be possible for the light client to execute runtime API calls without having the extra information provided by the runtimeUpdates flag?